1. Field of the Invention
The present invention relates to a control system for an automatic transmission to be mounted on an internal combustion engine in an automotive vehicle, and is applicable particularly to a control system for switching shift patterns in compliance with the temperature of hydraulic oil contained in a torque converter of the automatic transmission.
2. Description of the Prior Art
Recently, an automotive vehicle provided with an automatic transmission capable of selecting any one of a plurality of shift patterns suitable for various running conditions has been commercially available. The most appropriate shift pattern varies according to the running conditions on flat ground, in mountains, or in overcrowded cities in traffic in which acceleration and deceleration are frequently required.
Japanese Patent Publication (examined) No. 48-217 discloses a control system for an automatic transmission, in which a shift point is changed according to the temperature of engine cooling water, engine lubricating oil, an engine block, hydraulic oil of a torque converter or the like. Each of these parameters varies in temperature according to the running conditions of an automotive vehicle. The shift point is changed by changing the setting conditions for shifting upon receipt of a signal from an electric signal generating means for generating an electric signal responsive to the temperature of the above parameters.
In this control system, when the temperature of cooling water is low before much time elapses after engine starting, the running range covered by low speed gears is extended to the high speed side, thereby supplementing power reduction until the engine becomes warm.
More specifically, while the temperature of engine cooling water reaches an appropriate range after engine starting, the switching of shift patterns of the engine is controlled according to the temperature of the engine cooling water or the like. However, since a signal indicative of the temperature of the engine cooling water is generally supplied to an engine control unit for controlling the engine, this signal must be transmitted via a signal line, which communicates between the engine control unit and a transmission control unit. Because of this, the above conventional control system needs a lead wire for electrically connecting the engine control unit and the transmission control unit, thereby increasing the number of lead wires in a wire harness, complicating connectors thereof and increasing the manufacturing cost.
In the meantime, there is a correlation between the temperature of engine cooling water and that of hydraulic oil contained in a torque converter of an automatic transmission. In general, the temperature of hydraulic oil of the torque converter rises with a temperature rise of engine cooling water. In view of this correlation between the temperature of engine cooling water and that of hydraulic oil of the torque converter, it is possible to control the switching of shift patterns with the temperature of hydraulic oil of the torque converter being employed as control parameter for the transmission.
However, there is not always a correlation between the temperature of engine cooling water and that of hydraulic oil of the torque converter of the automatic transmission. This correlation would be lost, for example, in the following case.
When idling conditions are continued for more than a certain time period after engine starting, the temperature of engine cooling water rises but the temperature rise of hydraulic oil of the torque converter is relatively small.
Even in this case, if the switching of shift patterns is controlled on the basis of the temperature of hydraulic oil of the torque converter and if the idling conditions are continued for a long time after engine starting, the running range covered by low speed gears is unnecessarily extended to the high speed side The reason for this is that the transmission is controlled by a shift pattern favored for a cold engine, although the engine has become warm. As a result, the problem arose that the fuel consumption increased.